The present invention generally relates to portable telephone systems, and more particularly to a portable telephone system having a selectable bypass line.
FIG. 1 generally shows an example of a conventional portable telephone system. In FIG. 1, a telephone network 100 is connected to a portable telephone network 200 at a connection point POI via a communication center 20. The telephone network 100 is a public network. When making a call from a telephone set A which is connected to a terminal station EO of the telephone network 100 to a portable station PS of the portable telephone network 200, the calling telephone set A dials a personal identification number (hereinafter simply referred to as PID number) of the portable station, following number of the communication center 20. The call from the calling telephone set A is transmitted from the end terminal EO via a communication line which includes a toll center TC, a district center DC, a regional center RC, an interconnecting gateway switch IGS and the like, and first reaches the communication center 20 of the portable telephone network 200 via the connection point POI. Then, the communication center 20 searches the position of the portable station PS which is called. If the portable station PS is located within a service area 12 or 13 of the portable telephone network 200, the communication center 20 transfers the call from the telephone set A to a portable station PS.sub.2 within the service area 12 or, to a portable station PS.sub.3 within the service area 13, via a private line, so as to enable a communication by forming a talking line between the calling telephone set A and the portable station PS.sub.2 or PS.sub.3.
However, when the calling telephone set A which is connected to the terminal station EO of the telephone network 100 makes a call to the portable station PS, the portable station PS may not be located within the service area 12 or 13 of the portable telephone network 200. For example, the called portable station PS may be a portable station PS.sub.B which is connected by radio to a parent station CA of a telephone set B which is independently registered and connected to the terminal station EO of the telephone network 100. On the other hand, the called portable station PS may be a portable station PS.sub.1 which is connected by radio to a radio base station BS, where this radio base station BS is controlled by a switching control part CS.sub.1 which is connected to the telephone network 100 and is provided in a specific area 11, for example. The switching control part CS.sub.1 within the specific area 11 is not connected to the communication center 20 via a private line for economical reasons.
In such cases where the portable station PS.sub.B or PS.sub.1 is called, the call is transferred again from the connection point POI to the portable station PS.sub.B of the telephone set B or to the portable station PS.sub.1 within the specific area 11 via the communication line of the telephone network 100, and the call is made via the talking line between the telephone set A and the portable station PS.sub.B or PS.sub.1. For this reason, the communication line of the telephone network 100 is used twice, that is, once for the going path and once for the returning path, and there is a problem in that the cost of use of the communication line becomes high.